538 



PHYSIOLOGY. 



root is occupied by a bundle of n'bro-vascular tissue ; if they spring from 

 the stem opposite the pith or cellular tissue between the vessels, then 

 the centre of the root is likewise cellular. In Cryptogams, according to 

 Nageli, the roots always originate opposite a n'bro-vascular bundle. 



The primary form and disposition of the roots depend in a measure on 

 the form of the terminal cells and on the direction of the partitions by 

 which they are divided lengthwise, horizontal, or oblique. 



Fig. 584. 



a, epidermal cells ; b, c, d, root-hairs. 



Tubei-cras Roots of herbaceous Dicotyledons present several modifica- 

 tions in the arrangement of the structures. In the Carrot and Parsnep 

 the fibro-vascular ring has its component parts much separated by the 

 great development of the medullary rays and masses of parenchyma re- 

 placing the ordinary prosenchyma of woody roots, so that the fibro- 

 vascular structure has a deceptive resemblance to that of Monocotyledons ; 

 and the cortical parenchyma, again, is greatly developed, so as to form a 

 thick fleshy rind. In the Turnip the cortical parenchyma is little deve- 

 loped, and the mass of the fibro-vascular bundles lies immediately under 

 the rind, the inner vascular parts of the bundles being split up, as it were, 



